Hybrid-channel nano-sheet FETs

ABSTRACT

Semiconductor devices and methods of forming a first layer cap at ends of layers of first channel material in a stack of alternating layers of first channel material and second channel material. A second layer cap is formed at ends of the layers of second channel material. The first layer caps are etched away in a first device region. The second layer caps are etched away in a second device region. First source/drain regions are grown in the first device region from exposed ends of the layers of the first channel material. Second source/drain regions are grown in the second device region from exposed ends of the layers of the second channel material.

BACKGROUND Technical Field

The present invention generally relates to semiconductor devices and,more particularly, to forming multiple transistor devices on a singlesubstrate with different channel properties.

Description of the Related Art

As complementary metal-oxide semiconductor (CMOS) fabrication processeshave improved and scaled down, the structures involved have reachednanometer scales. Various different nano-scale device architectures havebeen used to push the boundaries of CMOS technologies, including inparticular nanosheet devices that use sheets of channel material havingthicknesses measured in nanometers. However, different channel materialsare needed for n-type field effect transistors as compared to p-typefield effect transistors to attain optimal performance.

SUMMARY

A method of forming semiconductor devices includes forming a first layercap at ends of layers of first channel material in a stack ofalternating layers of first channel material and second channelmaterial. A second layer cap is formed at ends of the layers of secondchannel material. The first layer caps are etched away in a first deviceregion. The second layer caps are etched away in a second device region.First source/drain regions are grown in the first device region fromexposed ends of the layers of the first channel material. Secondsource/drain regions are grown in the second device region from exposedends of the layers of the second channel material.

These and other features and advantages will become apparent from thefollowing detailed description of illustrative embodiments thereof,which is to be read in connection with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

The following description will provide details of preferred embodimentswith reference to the following figures wherein:

FIG. 1 is a cross-sectional diagram of a step in the formation of anintegrated chip in accordance with the present principles;

FIG. 2 is a cross-sectional diagram of a step in the formation of anintegrated chip in accordance with the present principles;

FIG. 3 is a cross-sectional diagram of a step in the formation of anintegrated chip in accordance with the present principles;

FIG. 4 is a cross-sectional diagram of a step in the formation of anintegrated chip in accordance with the present principles;

FIG. 5 is a cross-sectional diagram of a step in the formation of anintegrated chip in accordance with the present principles;

FIG. 6 is a cross-sectional diagram of a step in the formation of anintegrated chip in accordance with the present principles;

FIG. 7 is a cross-sectional diagram of a step in the formation of anintegrated chip in accordance with the present principles;

FIG. 8 is a cross-sectional diagram of a step in the formation of anintegrated chip in accordance with the present principles;

FIG. 9 is a cross-sectional diagram of a step in the formation of anintegrated chip in accordance with the present principles;

FIG. 10 is a cross-sectional diagram of a step in the formation of anintegrated chip in accordance with the present principles;

FIG. 11 is a cross-sectional diagram of a step in the formation of anintegrated chip in accordance with the present principles;

FIG. 12 is a cross-sectional diagram of a step in the formation of anintegrated chip in accordance with the present principles;

FIG. 13 is a cross-sectional diagram of a step in the formation of anintegrated chip in accordance with the present principles;

FIG. 14 is a cross-sectional diagram of a step in the formation of anintegrated chip in accordance with the present principles;

FIG. 15 is a cross-sectional diagram of a step in the formation of anintegrated chip in accordance with the present principles; and

FIG. 16 is a block/flow diagram of a method for forming an integratedchip in accordance with the present principles.

DETAILED DESCRIPTION

Embodiments of the present invention provide n-type field effecttransistors (NFETs) and p-type field effect transistors (PFETs) thathave different channel materials and device properties. To accomplishthis, a stack of alternating nanosheets is selectively patterned toexpose one or the other type of channel material when forming source anddrain regions.

Referring now to the drawings in which like numerals represent the sameor similar elements and initially to FIG. 1, a cross-sectional diagramof a step in forming nanosheet FETs having different channel propertiesis shown. A semiconductor substrate 102 is provided. The semiconductorsubstrate 102 may be a bulk-semiconductor substrate. In one example, thebulk-semiconductor substrate may be a silicon-containing material.Illustrative examples of silicon-containing materials suitable for thebulk-semiconductor substrate include, but are not limited to, silicon,silicon germanium, silicon germanium carbide, silicon carbide,polysilicon, epitaxial silicon, amorphous silicon, and multi-layersthereof. Although silicon is the predominantly used semiconductormaterial in wafer fabrication, alternative semiconductor materials canbe employed, such as, but not limited to, germanium, gallium arsenide,gallium nitride, cadmium telluride, and zinc selenide. Although notdepicted herein, the semiconductor substrate 102 may also be asemiconductor on insulator (SOI) substrate.

A stack of alternating semiconductor layers 108 is formed on thesemiconductor substrate 102. Layers of a first channel material 104alternate with layers of a second channel material 106. The first andsecond channel materials have etch selectivity with respect to oneanother. As used herein, the term “selective” in reference to a materialremoval process denotes that the rate of material removal for a firstmaterial is greater than the rate of removal for at least anothermaterial of the structure to which the material removal process is beingapplied. In one specific embodiment, etch selectivity denotes a ratio ofetch rates between an etched material and other materials of at least10:1.

In one specific embodiment, the first layers of channel material areformed from silicon germanium and the second layers of channel materialare formed from silicon. These two materials have etch selectivity withrespect to one another, such that the material of one set of layers canbe removed without substantially affecting the material of the otherlayers. It should be understood, however, that any appropriatecombination of semiconductor materials having etch selectivity withrespect to one another may be used instead.

The stack of alternating semiconductor layers 108 may be formed by anyappropriate deposition process, including for example chemical vapordeposition (CVD), atomic layer deposition (CVD), physical vapordeposition (PVD), and gas cluster ion beam (GCIB) deposition. CVD is adeposition process in which a deposited species is formed as a result ofchemical reaction between gaseous reactants at greater than roomtemperature (e.g., from about 25° C. about 900° C.). The solid productof the reaction is deposited on the surface on which a film, coating, orlayer of the solid product is to be formed. Variations of CVD processesinclude, but are not limited to, Atmospheric Pressure CVD (APCVD), LowPressure CVD (LPCVD), Plasma Enhanced CVD (PECVD), and Metal-Organic CVD(MOCVD) and combinations thereof may also be employed. In alternativeembodiments that use PVD, a sputtering apparatus may includedirect-current diode systems, radio frequency sputtering, magnetronsputtering, or ionized metal plasma sputtering. In alternativeembodiments that use ALD, chemical precursors react with the surface ofa material one at a time to deposit a thin film on the surface. Inalternative embodiments that use GCIB deposition, a high-pressure gas isallowed to expand in a vacuum, subsequently condensing into clusters.The clusters can be ionized and directed onto a surface, providing ahighly anisotropic deposition.

Referring now to FIG. 2, a cross-sectional diagram of a step in formingnanosheet FETs having different channel properties is shown. The figuredepicts an NFET region 202 and a PFET region 204. It should beunderstood that these two regions are formed on a single wafer and maybe separated by a substantial distance. Dummy gates 208 are formed inthe respective regions and may be formed from any appropriate materialsuch as, e.g., polysilicon or amorphous silicon and are separated fromthe underlying stack of alternating semiconductor layers 108 by aninsulator pad 209. The dummy gates 208 are surrounded by an insulatorlayer formed from, e.g., a gate pattern hardmask 210 and a spacer 206.It is specifically contemplated that the gate pattern hardmask 210 maybe formed from any appropriate hardmask material such as, e.g., siliconnitride, and that the spacer 206 may be formed from any appropriatedielectric material including, e.g., silicon boron carbon nitride. Itshould be noted that the materials of the spacer 206 and the hardmask210 should be selectively etchable with respect to one another.

The dummy gates 208 may be formed by depositing a dielectric hard maskmaterial, such as silicon nitride or silicon dioxide, on a layer ofdummy gate material and then applying a photoresist pattern to the hardmask material using a lithography process. The photoresist pattern isthen transferred into the hard mask material using, e.g., a dry etchprocess to form the gate pattern hardmask 210. Next, the photoresistpattern is removed and the gate pattern is then transferred into thedummy gate material during an anisotropic selective etching process,such as reactive ion etching (RIE). Alternatively, the first and secondgate structure 6 can be formed by other patterning techniques such asspacer image transfer.

RIE is a form of plasma etching in which during etching the surface tobe etched is placed on a radio-frequency powered electrode. During RIEthe surface to be etched takes on a potential that accelerates theetching species extracted from plasma toward the surface, in which thechemical etching reaction is taking place in the direction normal to thesurface. Other examples of anisotropic etching that can be used at thispoint of the present invention include ion beam etching, plasma etchingor laser ablation.

Referring now to FIG. 3, a cross-sectional diagram of a step in formingnanosheet FETs having different channel properties is shown. Ananisotropic etch is applied that stops on the substrate 102. Theanisotropic etch should remove material from the spacer 206, such thatit can penetrate to the underlying stack of layers 108, but should etchthe stack of semiconductor layers 108 at a rate greater than the rate atwhich it etches the spacer 206 and the hardmask 210. This prevents theetch from damaging the dummy gate 208.

It is specifically contemplated that RIE may be used to perform theanisotropic etch. In one specific embodiment, multiple different etchesmay be used, with a first etch anisotropically removing material toremove material around the thicker gate pattern hardmask 206 andvertical spacer, followed by a second etch that is selective to thematerials of the stack 108 without affecting the remaining hardmaskmaterials. What remains is etched channel stacks 302 in the NFET regions202 and the PFET regions 204, with the etched channel stacks 302extending no farther than the remaining gate spacers 304.

Referring now to FIG. 4, a cross-sectional diagram of a step in formingnanosheet FETs having different channel properties is shown. Anisotropic etch is used to laterally etch the first layers of channelmaterial 104. The etch selectively removes material from the firstlayers of channel material 104 and leaves the material of the secondlayers of channel material in place 106, creating etched first channelmaterial layers 402.

Referring now to FIG. 5, a cross-sectional diagram of a step in formingnanosheet FETs having different channel properties is shown. Adielectric layer is conformally formed over the structures andsubsequently etched away with an isotropic etch. In one specificexample, CVD may be used to form the dielectric material. This formsdielectric plugs 502 at the exposed ends of the etched first channelmaterial layers 402. One exemplary embodiment creates cavities that are,e.g., 6 nm high, filled with a 4 nm conformal dielectric layer thatpinches off in the cavity. The isotropic etch then removes material frommost surfaces, but the material will be thicker inside the cavities,such that the dielectric plugs 502 remain.

Referring now to FIG. 6, a cross-sectional diagram of a step in formingnanosheet FETs having different channel properties is shown. The exposedsurfaces of the second layers of channel material 106 are oxidized toform oxide caps 602. In the specific example where the second layers ofchannel material 106 and the substrate 102 are both formed from silicon,this step produces a silicon dioxide layer on all exposed siliconsurfaces. It should be understood, however, that other embodiments makeuse of channel materials other than silicon and silicon germanium. Assuch, the formation of the plugs/caps 502 and 602 may involve otherprocesses with the effect of creating selectively etchable structures atthe ends of the respective channel nanosheets.

Referring now to FIG. 7, a cross-sectional diagram of a step in formingnanosheet FETs having different channel properties is shown. The PFETregion 204 is masked with any appropriate masking material 702, leavingthe NFET region 202 exposed. The oxide material is removed, leaving endsof the etched second channel layers 704 exposed. It is specificallycontemplated that the masking material 702 may be a polymer material.

Referring now to FIG. 8, a cross-sectional diagram of a step in formingnanosheet FETs having different channel properties is shown. The mask702 is removed and source/drain regions 802 are epitaxially grown fromthe exposed ends of the etched second channel layers 704. It isspecifically contemplated that the source/drain regions 802 may be dopedin situ, but alternative embodiments may include doping by implantationthat is performed before the mask 702 is removed. Any appropriate dopantmay be selected for the source/drain regions 802 of the device in theNFET region 202.

In particular, an n-type dopant may be used to dope the source/drainregions 802. As used herein, “n-type” refers to the addition ofimpurities that contributes free electrons to an intrinsic semiconductorin a silicon containing substrate examples of n-type dopants, i.e.,impurities, include but are not limited to antimony, arsenic andphosphorous. The dopant concentration in the source/drain region 802 canrange from—about 1×10¹⁹ cm⁻³ to about 2×10²¹ cm⁻³, or preferably between2×10²⁰ cm⁻³ and 1×10²¹ cm³.

Referring now to FIG. 9, a cross-sectional diagram of a step in formingnanosheet FETs having different channel properties is shown. A layer ofspacer material is deposited over the nFET region 202 and the pFETregion 204. The spacer material may be formed from, e.g., silicon carbonoxide or any other appropriate sacrificial material with etchselectivity to the other structures. A layer of mask material (e.g., apolymer) is then deposited over the nFET region 202 and the pFET region204. It is specifically contemplated that at least the layer of spacermaterial may be deposited conformally. The sacrificial spacer layer andthe layer of mask material are etched away in the pFET region 204 toexpose the underlying pFET structures.

Referring now to FIG. 10, a cross-sectional diagram of a step in formingnanosheet FETs having different channel properties is shown. Thedielectric plugs 502 are etched away using, e.g., an isotropic etch suchas a wet or dry chemical etch that selectively removes the plugs 502without affecting the other structures.

Referring now to FIG. 11, a cross-sectional diagram of a step in formingnanosheet FETs having different channel properties is shown. The mask904 is etched away before pFET source/drain regions 1102 are grown inthe pFET region 204 from the exposed ends of the etched first channelmaterial layers 402. It is specifically contemplated that thesource/drain regions 1102 may be doped in situ, but alternativeembodiments may include doping by implantation. Any appropriate dopantmay be selected for the source/drain regions 1102 of the device in thepFET region 204. After formation of the pFET source/drain regions 1102,the sacrificial spacer layer 902 is removed.

As used herein, “p-type” refers to the addition of impurities to anintrinsic semiconductor that creates deficiencies of valence electrons.In a silicon-containing substrate, examples of p-type dopants, i.e.,impurities, include but are not limited to: boron, aluminum, gallium andindium. The dopant concentration in the source/drain region 1102 canrange from about 1×10¹⁹ cm⁻³ to about 2×10²¹ cm⁻³, or preferably between2×10²⁰ cm⁻³ and 1×10²¹ cm⁻³.

Referring now to FIG. 12, a cross-sectional diagram of a step in formingnanosheet FETs having different channel properties is shown. A layer ofdielectric material 1202 is formed over the nFET region 202 and the pFETregion 204. It is specifically contemplated that the layer of dielectricmaterial 1202 may be formed by depositing, e.g., a flowable oxidematerial such as silicon dioxide. The deposited dielectric material isthen polished down to the level of the remaining hardmask 210 using,e.g., chemical mechanical planarization (CMP).

CMP is performed using, e.g., a chemical or granular slurry andmechanical force to gradually remove upper layers of the device. Theslurry may be formulated to be unable to dissolve, for example, thematerial of the hardmask 210, resulting in the CMP process's inabilityto proceed any farther than that layer.

Referring now to FIG. 13, a cross-sectional diagram of a step in formingnanosheet FETs having different channel properties is shown. Thehardmask 210 is etched away, followed by the dummy gate 208 and dummygate dielectric 209 in the nFET region 202. In addition, the remainingfirst channel material layers 402 in the nFET region 202 are etchedaway, leaving the etched second channel layers 704 exposed. The removalof these layers may be performed using one or more etches, but it shouldbe noted that an isotropic etch of the hardmask 210, dummy gate 208, anddummy gate dielectric 209 in the nFET region 202 may be preceded bymasking the pFET region 204 to prevent damage to correspondingstructures in the pFET region 204.

Referring now to FIG. 14, a cross-sectional diagram of a step in formingnanosheet FETs having different channel properties is shown. An n-typework function metal layer 1402 is deposited around the second channellayers 704 in the nFET region 202. Although it is not shown in thisfigure, a layer of work function metal will also be deposited along thevertical sidewalls of the gap. A gate contact 1404 is deposited over then-type work function metal layer 1402. It is specifically contemplatedthat the gate contact 1404 may be formed from tungsten, but anyappropriate conductive material may be used instead.

As used herein, an “n-type work function metal layer” is a metal layerthat effectuates an n-type threshold voltage shift. As used herein,“threshold voltage” is the lowest attainable gate voltage that will turnon a semiconductor device, e.g., transistor, by making the channel ofthe device conductive. “N-type threshold voltage shift” as used hereinmeans a shift in the Fermi energy of an n-type semiconductor devicetowards a conduction band of silicon in a silicon-containing substrateof the n-type semiconductor device. The “conduction band” is the lowestlying electron energy band of the doped material that is not completelyfilled with electrons. In one embodiment, the work function of then-type work function metal layer ranges from 4.1 eV to 4.3 eV. In oneembodiment, the n-type work function metal layer is composed of at leastone of titanium aluminum, tantalum nitride, titanium nitride, hafniumnitride, hafnium silicon, or combinations thereof. The n-type workfunction metal layer can be deposited using CVD, ALD, or plating.

Referring now to FIG. 15, a cross-sectional diagram of a step in formingnanosheet FETs having different channel properties is shown. As with thenFET region 202, the hardmask 210, dummy gate 208, and dummy gatedielectric 209 are removed from the pFET region 204, along withremaining portions of the second channel material layers 106 to exposethe etched first channel material layers 402. A p-type work functionmetal layer 1502 is deposited around the first channel material layers402. A gate contact 1504 is deposited over the n-type work functionmetal layer 1402. As with gate contact 1404, it is specificallycontemplated that the gate contact 1504 may be formed from tungsten, butany appropriate conductive material may be used instead.

As used herein, a “p-type work function metal layer” is a metal layerthat effectuates a p-type threshold voltage shift. In one embodiment,the work function of the p-type work function metal layer ranges from4.9 eV to 5.2 eV. The term “p-type threshold voltage shift” as usedherein means a shift in the Fermi energy of a p-type semiconductordevice towards a valence band of silicon in the silicon containingsubstrate of the p-type semiconductor device. A “valence band” is thehighest range of electron energies where electrons are normally presentat absolute zero.

It is to be understood that aspects of the present invention will bedescribed in terms of a given illustrative architecture; however, otherarchitectures, structures, substrate materials and process features andsteps can be varied within the scope of aspects of the presentinvention.

It will also be understood that when an element such as a layer, regionor substrate is referred to as being “on” or “over” another element, itcan be directly on the other element or intervening elements can also bepresent. In contrast, when an element is referred to as being “directlyon” or “directly over” another element, there are no interveningelements present. It will also be understood that when an element isreferred to as being “connected” or “coupled” to another element, it canbe directly connected or coupled to the other element or interveningelements can be present. In contrast, when an element is referred to asbeing “directly connected” or “directly coupled” to another element,there are no intervening elements present.

The present embodiments can include a design for an integrated circuitchip, which can be created in a graphical computer programming language,and stored in a computer storage medium (such as a disk, tape, physicalhard drive, or virtual hard drive such as in a storage access network).If the designer does not fabricate chips or the photolithographic masksused to fabricate chips, the designer can transmit the resulting designby physical means (e.g., by providing a copy of the storage mediumstoring the design) or electronically (e.g., through the Internet) tosuch entities, directly or indirectly. The stored design is thenconverted into the appropriate format (e.g., GDSII) for the fabricationof photolithographic masks, which typically include multiple copies ofthe chip design in question that are to be formed on a wafer. Thephotolithographic masks are utilized to define areas of the wafer(and/or the layers thereon) to be etched or otherwise processed.

Methods as described herein can be used in the fabrication of integratedcircuit chips. The resulting integrated circuit chips can be distributedby the fabricator in raw wafer form (that is, as a single wafer that hasmultiple unpackaged chips), as a bare die, or in a packaged form. In thelatter case, the chip is mounted in a single chip package (such as aplastic carrier, with leads that are affixed to a motherboard or otherhigher level carrier) or in a multichip package (such as a ceramiccarrier that has either or both surface interconnections or buriedinterconnections). In any case, the chip is then integrated with otherchips, discrete circuit elements, and/or other signal processing devicesas part of either (a) an intermediate product, such as a motherboard, or(b) an end product. The end product can be any product that includesintegrated circuit chips, ranging from toys and other low-endapplications to advanced computer products having a display, a keyboardor other input device, and a central processor.

It should also be understood that material compounds will be describedin terms of listed elements, e.g., SiGe. These compounds includedifferent proportions of the elements within the compound, e.g., SiGeincludes Si_(x)Ge_(1-x) where x is less than or equal to 1, etc. Inaddition, other elements can be included in the compound and stillfunction in accordance with the present principles. The compounds withadditional elements will be referred to herein as alloys.

Reference in the specification to “one embodiment” or “an embodiment”,as well as other variations thereof, means that a particular feature,structure, characteristic, and so forth described in connection with theembodiment is included in at least one embodiment. Thus, the appearancesof the phrase “in one embodiment” or “in an embodiment”, as well anyother variations, appearing in various places throughout thespecification are not necessarily all referring to the same embodiment.

It is to be appreciated that the use of any of the following “/”,“and/or”, and “at least one of”, for example, in the cases of “A/B”, “Aand/or B” and “at least one of A and B”, is intended to encompass theselection of the first listed option (A) only, or the selection of thesecond listed option (B) only, or the selection of both options (A andB). As a further example, in the cases of “A, B, and/or C” and “at leastone of A, B, and C”, such phrasing is intended to encompass theselection of the first listed option (A) only, or the selection of thesecond listed option (B) only, or the selection of the third listedoption (C) only, or the selection of the first and the second listedoptions (A and B) only, or the selection of the first and third listedoptions (A and C) only, or the selection of the second and third listedoptions (B and C) only, or the selection of all three options (A and Band C). This can be extended, as readily apparent by one of ordinaryskill in this and related arts, for as many items listed.

The terminology used herein is for the purpose of describing particularembodiments only and is not intended to be limiting of exampleembodiments. As used herein, the singular forms “a,” “an” and “the” areintended to include the plural forms as well, unless the context clearlyindicates otherwise. It will be further understood that the terms“comprises,” “comprising,” “includes” and/or “including,” when usedherein, specify the presence of stated features, integers, steps,operations, elements and/or components, but do not preclude the presenceor addition of one or more other features, integers, steps, operations,elements, components and/or groups thereof.

Spatially relative terms, such as “beneath,” “below,” “lower,” “above,”“upper,” and the like, can be used herein for ease of description todescribe one element's or feature's relationship to another element(s)or feature(s) as illustrated in the FIGS. It will be understood that thespatially relative terms are intended to encompass differentorientations of the device in use or operation in addition to theorientation depicted in the FIGS. For example, if the device in theFIGS. is turned over, elements described as “below” or “beneath” otherelements or features would then be oriented “above” the other elementsor features. Thus, the term “below” can encompass both an orientation ofabove and below. The device can be otherwise oriented (rotated 90degrees or at other orientations), and the spatially relativedescriptors used herein can be interpreted accordingly. In addition, itwill also be understood that when a layer is referred to as being“between” two layers, it can be the only layer between the two layers,or one or more intervening layers can also be present.

It will be understood that, although the terms first, second, etc. canbe used herein to describe various elements, these elements should notbe limited by these terms. These terms are only used to distinguish oneelement from another element. Thus, a first element discussed belowcould be termed a second element without departing from the scope of thepresent concept.

Referring now to FIG. 16, a method for forming nanosheet FETs havingdifferent channel properties is shown. Block 1602 forms alternatingchannel layers 104 and 106 (e.g., alternating between silicon andsilicon germanium) on a substrate 102. Block 1604 forms dummy gatestructures over the stack 108 of alternating channel layers in a firstdevice region 202 and a second device region 204. Block 1606 then trimsthe channel layers in the stack 108 that are not covered by the dummygate structures using, e.g., an anisotropic etch.

Block 1608 recesses the first channel layers 104 using an isotropic etchthat removes material laterally while leaving the second channel layers106 relatively undisturbed. Block 1610 then fills in the recesses with adielectric material. Block 1612 oxidizes the ends of the second channellayers 106.

Block 1614 removes the oxidized material from a first device region 202that corresponds to, e.g., nFET devices. Block 1616 grows source/drainregions 802 from the exposed ends of the second channel layers 106. Thesecond device region 204 may be masked during the etching and growthprocesses in the first device region 202.

Block 1618 removes the dielectric fill at the ends of the first channellayers 402 in the second device region 204. Block 1620 then growssource/drain regions 1102 from the exposed ends of the first channellayers 402. The first device region 202 may be masked during the etchingand growth processes.

Block 1622 forms dielectric layer 1202 over both device regions by,e.g., depositing a flowable oxide material and polishing the resultinglayer down to the level of the dummy gate structure using a CMP process.Block 1624 forms gate structures in each of the device regions, byremoving the dummy gate structures and either the first or secondchannel layer materials from each of the device regions. The final gatestructures are formed by depositing a respective work function metallayer in each device region and a gate contact in contact with the workfunction metal layer. It is specifically contemplated that the workfunction metal layers will be different in the two device regions suchthat the devices will have different properties. For example an n-typework function metal layer 1402 may be deposited in the first deviceregion 202 and a p-type work function metal layer 1502 may be depositedin the second device region 204. Contacts may be formed to therespective source and drain regions by any appropriate process, formingelectrical contacts to said regions.

Having described preferred embodiments of a system and method (which areintended to be illustrative and not limiting), it is noted thatmodifications and variations can be made by persons skilled in the artin light of the above teachings. It is therefore to be understood thatchanges may be made in the particular embodiments disclosed which arewithin the scope of the invention as outlined by the appended claims.Having thus described aspects of the invention, with the details andparticularity required by the patent laws, what is claimed and desiredprotected by Letters Patent is set forth in the appended claims.

What is claimed is:
 1. A method of forming semiconductor devices:forming first layer caps at ends of layers of first material in stacksof alternating layers of first material and second material; formingsecond layer caps at ends of the layers of second material; etching awaythe first layer caps in only a first device region; etching away thesecond layer caps in only a second device region; growing firstsource/drain regions in the first device region from exposed ends of thelayers of the first material; and growing second source/drain regions inthe second device region from exposed ends of the layers of the secondmaterial.
 2. The method of claim 1, wherein the first material issilicon and wherein the second material is silicon germanium.
 3. Themethod of claim 1, wherein the first source/drain regions are n-typedoped and wherein the second source/drain regions are p-type doped. 4.The method of claim 1, further comprising forming dummy gate structureson the stacks of alternating layers of first material and secondmaterial in the first and second device regions.
 5. The method of claim4, further comprising etching away material of the stacks of alternatinglayers of first material and second material in areas that are notcovered by the dummy gate structures.
 6. The method of claim 4, furthercomprising etching away the dummy gate structures after growing thefirst and second source/drain regions.
 7. The method of claim 6, furthercomprising etching away the layers of second material in the firstdevice region and etching away the layers of first material in thesecond device region.
 8. The method of claim 7, further comprisingdepositing a first work function metal layer around the layers of firstmaterial in the first device region and depositing a second workfunction metal layer around the layers of second material in the seconddevice region.
 9. The method of claim 1, wherein forming the first layercaps comprises oxidizing exposed portions of the first material.
 10. Themethod of claim 1, wherein forming the second layer caps compriseslaterally etching the layers of the second material to form recesses anddepositing a dielectric fill in the recesses.
 11. The method of claim 1,wherein the first layer caps are formed only on the layers of firstmaterial and the second layer caps are formed only on the layers ofsecond material.
 12. A method of forming semiconductor devices: formingfirst layer caps at ends of layers of first material in stacks ofalternating layers of first material and second material by oxidizingexposed portions of the first material; forming second layer caps atends of the layers of second material; etching away the first layer capsin a first device region; etching away the second layer caps in a seconddevice region; growing first source/drain regions in the first deviceregion from exposed ends of the layers of the first material; andgrowing second source/drain regions in the second device region fromexposed ends of the layers of the second material.
 13. The method ofclaim 12, further comprising forming dummy gate structures on the stacksof alternating layers of first material and second material in the firstand second device regions.
 14. The method of claim 13, furthercomprising etching away the dummy gate structures after growing thefirst and second source/drain regions, etching away the layers of secondmaterial in the first device region, and etching away the layers offirst material in the second device region.
 15. The method of claim 12,wherein forming the second layer caps comprises laterally etching thelayers of the second material to form recesses and depositing adielectric fill in the recesses.
 16. A method of forming semiconductordevices: forming first layer caps at ends of layers of first material instacks of alternating layers of first material and second material;forming second layer caps at ends of the layers of second material bylaterally etching the layers of the second material to form recesses anddepositing a dielectric fill in the recesses; etching away the firstlayer caps in a first device region; etching away the second layer capsin a second device region; growing first source/drain regions in thefirst device region from exposed ends of the layers of the firstmaterial; and growing second source/drain regions in the second deviceregion from exposed ends of the layers of the second material.
 17. Themethod of claim 16, further comprising forming dummy gate structures onthe stacks of alternating layers of first material and second materialin the first and second device regions.
 18. The method of claim 17,further comprising etching away the dummy gate structures after growingthe first and second source/drain regions, etching away the layers ofsecond material in the first device region, and etching away the layersof first material in the second device region.
 19. The method of claim16, wherein forming the first layer caps comprises oxidizing exposedportions of the first material.